This invention relates to the use of attenuated total reflectance (ATR) elements (e.g., probes) which provide spectral analysis of liquid samples. It relates particularly to the spectral analysis of liquids which require higher than usual angles of incidence of the rays which are reflected inside the ATR probes, and are subject to partial absorption by the sample.
Attenuated total reflectance is a widely used technique for spectrally analyzing liquids having absorptions which are too strong for convenient transmission analysis. This condition is commonly encountered in the infrared (IR) region of the spectrum--the spectral region which includes the fundamental frequencies or most molecular vibrations. The ATR has also found some use in the ultra-violet (UV) and visible regions, as in the analysis of dyes and other strongly absorbing substances which are soluble in water.
Over the years, a large number of ATR devices have been developed--including several by the present inventor. ATR probes generally fall into two categories: those employing cylindrical ATR rods (see FIG. 2 of U.S. Pat. No. 5,051,551), and those employing conical elements (see FIG. 8 of the same patent).
Both of these figures show optical rays which strike the interface between the ATR element and the analyte (sample) at an incidence angle of 45 degrees. The incidence angle is defined as the angle between the ray direction and the normal to the surface. A 45 degree incidence angle is often convenient. That angle is usually appropriate for the analysis of organics in the IR region. For the high index ATR materials available for use in the IR region, it is sufficiently above the critical angle to avoid significant data distortion.
The critical angle is defined as the smallest incidence angle for which an optical ray, in the absence of absorption, will be totally reflected at the interface. It is equal to EQU .phi..sub.c =sin.sup.-1 (n.sub.1 /n.sub.2)
where n.sub.1 and n.sub.2 represent the indices of refraction of the analyte and the ATR element, respectively. Typical organic materials have refractive indices around 1.5, while several common infrared ATR materials have indices equal to 2.4 or higher. For this particular combination, the critical angle is equal to 38.7.degree.. The 45.degree. angle often used is well beyond this.
For some applications, it is desirable-or even necessary-to use an incident angle larger than 45.degree.. This can occur, even when using a relatively high index ATR material, if the analyte is very strongly absorbing, or has an especially high refractive index. In other cases, the use of a high index ATR material may be precluded by such considerations as vulnerability to chemical attack, or required spectral region for the analysis. For example, in the higher frequency portion of the IR region, both sapphire (n=1.8) and cubic zirconia (n-2.0) provide good chemical resistance and high optical transmission. But their relatively low indices necessitate the use of incidence angles greater than 45.degree.. With the rod type ATR probe, this can be accomplished by altering the included angle of the rod end cones and/or the angle of the metallic reflecting cones. However, with the conical element ATR probe design, the situation isn't as simple.
Conical element ATR probes have a distinct advantage over the rod design for many industrial applications, in that they provide much higher transmission for a given probe diameter. However, since the ATR element also functions as a retroreflector, it is not possible to change the cone angle at will. One solution for this problem is to use more than two reflections. Examples corresponding to three and four reflections are shown in FIGS. 1 and 2. Three reflections with equal incidence angles will provide an incidence angle of 60.degree. (see FIG. 1), while four reflections will result in 67.5.degree. (see FIG. 2). A disadvantage of this approach is that, for a given incident beam diameter, the required probe diameter increases rapidly as the number of reflections increases.
Three reflection probes employing sapphire elements have been used for the UV and visible analysis of water solutions of strongly absorbing substances such as dyes. Since water has a low index (n=1.33), the critical angle is well below 60.degree.: EQU .phi..sub.c =sin.sup.-1 (1.33/1.8)=47.6.degree.
The difficulty arises when one attempts to use these same probes to analyze organic materials, many of which have very strong absorbances in the UV spectral region. Such materials often have average refractive indices around 1.5. With an ATR element having an index of 1.8, the critical angle will be 56.4.degree.. While this is still below 60.degree., it is close enough to give rise to significant band distortion, especially when one takes into account the range of angles which characterize a typical incoherent optical beam. But this isn't the whole story. Rather than remaining fixed at its average value, the refractive index of a substance departs substantially from this value in the vicinity of a strong absorption band. This situation is illustrated by FIGS. 3a, 3b, and 3c. Since the index is elevated on the longer wavelength side of the band, the critical angle will be higher and the penetration depth greater. The result will be a skewing of the measured band (see FIG. 3c). The only solution to this problem is a further increase in incidence angle.
As noted earlier, one solution to the above problem would be to use a four reflection ATR element as shown in FIG. 2. However, this would substantially increase the required probe diameter for a given input beam area. It would also increase the travel distance of the beam in the element, giving rise to greater signal loss due to beam spread (vignetting) and possibly bulk absorption.